Sharp tip in texture mode
Circle brush

You can use Curve to change your tip sharpness. There are six presets for them and you can make your own as well.

Curve types:



Square brush

The best way to have square tip or any other shape is to prepare one in 2D software (Krita, Gimp, InkScape, Adobe etc.) 

Make tip shape white and transparent background (for convenience sake) make it pretty big for good quality (4k in my example) and save it as PNG with Transparency then setup Texture Mask as below:



Blend file (square tip texture packed)